Nmr studies designed to determine which functional groups in antibiotics of the tetracycline series are involved in binding to biologically important metal ions will be continued. All data to date indicate binding occurs at ring A groups, however the specific groups involved have not yet been established. Current work and work planned over the next year will be directed toward this goal. Metal binding properties of tetracycline derivatives in which ring A functional groups are altered or removed will be compared with metal binding in normal tetracycline. Also efforts will be made to understand the Mg(II) complex of tetracycline, which appears to exist in a conformation different from that of tetracycline in the presence of other metal ions.